a) Field of the Invention
The present invention concerns polyionic compounds, the process of preparation and use thereof as photoinitiators for the cationic polymerization or cross-linking of monomers and prepolymers, or for the modification of the solubility parameters of certain polymers which can be used as photoresists.
b) Description of Prior Art
A polymerization which involves a mechanism of the cationic type has many advantages. In particular, it is fast, even at low temperature, the coefficient of utilization of the monomer is high and the sensitivity towards atmospheric contaminants, such as oxygen, is low as compared to radical or anionic polymerizations.
Monomers, prepolymers and polymers containing cycloaliphatic epoxy functions, and vinyl ethers are increasingly used, for example in the paint, varnish, ink, glue, and antiadhesive support industries. Moreover, vinyl ethers generally appear to be non-toxic as opposed to acrylates and methacrylates. Monomers and prepolymers of the epoxy or vinyl ether types may be polymerized according to various methods, ionic polymerization being particularly preferred.
Cationic polymerization catalysts are generally considered as being acids within the meaning of Bronsted HX (proton donors) or as being acids within the meaning of Lewis (acceptors of electronic doublets), the latter operating in the presence of a co-catalyst which is a source of protons. These acids may be sufficiently strong to ensure the stability of the cationic species which is carried either by the monomer or by the growing macromolecular chain, which means that the corresponding anion X.sup.- should have a nucleophilic power which is as low as possible. The Bronsted acids which are most commonly used as cationic polymerization catalysts are CF.sub.3 SO.sub.3 H, HClO.sub.4, HBF.sub.4, HPF.sub.6, HAsF.sub.6 and HSbF.sub.6. These acids are classified as follows with respect to speeds of initiation of propagation as well as of obtention of the highest molecular weights: EQU CF.sub.3 SO.sub.3 H&lt;HClO.sub.4 .apprxeq.HBF.sub.4 &lt;HPF.sub.6 .apprxeq.HAsF.sub.6 .apprxeq.HSbF.sub.6.
More recently compounds with an acid character have also been used, such as bis(perfluoroalkylsulfonyl)imide (U.S. Pat. No. 4,031,036 Koshar et al.) or bis(perfluoroalkylsulfonyl)methane (U.S. Pat. No. 3,632,843 Allen et al).
It is known that the in situ preparation of polymerization catalysts has many advantages. The in situ production of an acid which is capable of catalyzing the cross-linking of a monomer enables indeed to obtain a monomer or a fluid prepolymer (thermoplastic material or solution) and to give it its final properties, for example by the simple action of a radiation. This technique is very much in use for inks, paints, adhesive films and anti-adhesive films. It should also be noted that the in situ preparation of the acid from a salt, in many cases enables to dispense with the stocking and handling of acid compounds which are more corrosive than the corresponding salts.
The catalysts may be prepared in situ by heat treatment. For example, ammonium or metal salts of bis(perfluoroalkylsulfonyl)imide (U.S. Pat. No. 4,031,036 Koshar et al.) or ammonium or amine salts of bis(perfluoroalkylsulfonyl)methane (U.S. Pat. No. 3,632,843 Allen et al.) have been used to obtain in situ, by heating, bis(perfluoroalkylsulfonyl)imide or the corresponding bis(perfluoroalkylsulfonyl)methane which thereafter act as catalysts. These catalysts called "latent", present, however, only a limited interest because of the necessity of extended heating at high temperature to achieve the removal of the acid, this removal being in addition progressive and not integral at the start. The result is that, on the one hand, the reaction speed is slow and, on the other hand, the polymers obtained are of poor quality with respect to molecular mass, polydispersity and coloring.
Acid catalysts may also be prepared in situ by actinic radiation (such as photons in which the wavelength corresponds to ultraviolet, visible, .gamma. and X radiation), or with .beta.-radiation (beam of electrons) on a suitable salt. Such salt, which is chemically labile under the action of an actinic or .beta.-radiation which reduces the release of the corresponding acid having a strong catatalytic activity, is a photoinitiator. The advantages of such a process are numerous: the release of the catalyst by radiation is rapid and practically complete which results in a simultaneous initiation of the growth of the chains and therefore a more homogeneous distribution of the masses with less polydispersity and better mechanical properties. Polymerization may be carried out at a relatively low temperature which prevents the decomposition or coloring of the materials obtained, as well as the formation of bubbles when a solvent is used or when the reaction mixture contains a volative additive which is designed to be maintained in the final material and which plays the role of plasticizing agent.
The capacity of different salts (hexafluoroantimonates, hexafluoroarseniates, hexafluoroplatinates and tetrafluoroborates) of aryldiazonium, aryliodonium, arylsulfonium, arylacylsulfonium or areneferrocenium to form acids (respectively HSbF.sub.6, HAsF.sub.6, HPF.sub.6, HBF.sub.4), under the action of actinic radiation, which can be used as cationic polymerization catalysts, is known. However, all these salts have a toxicity which is not negligible and which is mainly associated with the central element of the anion part, Sb, As, P and B, as well as with the fluorine ions which may be released during the reaction of photolysis or during a later treatment of the polymer (fusion, extrusion . . . ). To fix a general value, diphenyl-iodonium hexafluoroantimonate has an LD50 of 40 mg/kg (measured according to Test No. 10929 TAR) and falls in the category of products classified as "highly toxic".
Other salts containing cations of the same nature but containing less toxic anions have then been proposed. Thus, U.S. Pat. No. 5,554,664 describes salts in which the anion is selected from tris(alkylsulfonyl)methylides, tris(arylsulfonyl)-methylides, bis(alkylsulfonyl)imides and bis(arylsulfonyl)imides in which the alkyl group or the aryl group is perfluorinated or is highly fluorinated and whose cation is an iodoniun, a sulfonium or an organometallic. These compounds may be used, for example, as polymerization initiators after activation in situ. When these salts are used as polymerization photoinitiators, they leave, after decomposition initiated by actinic radiation, fragments which may diffuse at the surface of the material and modify the chemical properties, adhesiveness or appearance in a very notable manner. In the case of sulfonium salts, these residues contain thiols and thioethers in which the repulsive odor is perceptible at extremely low rates, which limits the use of these salts to particular applications. These compounds are also corrosive towards metals, such as copper or various components used in microelectronics. Thus, Kukzynski (U.S. Pat. No. 5,550,171) considers that the diffusion of the catalytic residues is the main cause of failure of computer storage discs.
Photosensitive polymers produced by the association of a polydiazonium polycation and a polysulfonate polyanion are described in U.S. Pat. No. 5,527,655. Such polymers are used to increase cross-linking efficiency. The solubility of these complexes is only obtained with diazodium contents clearly lower than 10% by weight and only in the presence of quartenary ammonium salts which are used for decreasing electrostatic interactions. U.S. Pat. No. 5,534,623 describes a composition based on polydiazoniun associated with contra-ions of the type PF.sub.6 intended for the preparation of photoresists. These anions are, however, toxic and are not compatible with microelectronics because they contain an element which is capable of contaminating silicon (B, P, As or Sb).
It is also known to use acids produced by means of actinic radiation in order to degrade the resins contained in a film constituting a photoresist. This technique is particularly efficient for photoresists with chemical amplification in which very small quantities of protons catalyze the decomposition of groups, such as esters containing a group derived from a tertiary alcohol (such as for example a tertiobutyl group) which is part of a macromolecular chain. This technique thus enables to modify the solubility parameters of the resin exposed to actinic radiation and to carry out masking and selective engraving operations such as those used in microelectronics.
When a photoresist composition or a resin composition with chemical amplification used in microlithography contains a photoinitiator, it is considered that the diffusion of the ionic species of the initiator or of the acid formed determines the limit of spatial resolution which is many tens of microns with non-polymer initiators. Now, actually resolutions lower than 1 micron are requested for the electronic industry of microprocessors and memories.